Many methods for separating biomolecules from mixtures such as cell lysates or synthetic preparations are based on a procedure in which the sample is loaded onto a column packed with a solid phase.
In the case of nucleic acids, for example, the solid phase can include an anion-exchange medium or resin. The negatively charged, anionic phosphate backbone of a nucleic acid can bind to and is thereby effectively immobilized by the resin. The resin can be washed with a low salt solution (e.g., 0.2 M sodium chloride), which flushes away the neutral, cationic, and less highly charged anionic components of the original mixture without substantially disrupting the binding of the nucleic acid molecules to the solid phase.
A high salt buffer solution (e.g., a buffer containing 1 M sodium chloride) is then used to elute the nucleic acid molecules away from the solid phase. The high salt concentration, however, can interfere with mass spectroscopy, electrophoresis, and many downstream enzymatic processes commonly employed in the laboratory or clinic, for example, for diagnostics, forensics, or genomic analysis. It is therefore necessary, in many cases, to remove at least some of the salt from the nucleic acid in an additional, frequently time-consuming step. Desalting can be accomplished by any of several procedures, including ethanol precipitation, dialysis, and purification from glass or silica beads or resin. In some cases it may also be necessary to add nuclease inhibitors to the wash and buffer solutions to prevent degradation of the nucleic acid.